It is necessary to inspect circuit components and their mountings to check for the presence of defects or in order to determine the cause of the defects. One known technique for inspecting the internal structure of integrated circuit components and their mountings onto printed circuit boards is X-ray imaging. X-rays from an X-ray source are passed through a region of interest on a circuit board and the resulting X-ray image or projection is detected by an X-ray detector on the opposite side of the circuit board to the X-ray source. This produces a two-dimensional image or slice through the region of interest. The X-ray source produces X-rays of sufficient energy to pass through the region of interest while also having a low enough energy to produce significant contrast within the resulting image.
This two-dimensional imaging technique is effective but often provides insufficient information. For example, there may be objects that occlude the desired view through the region of interest or it may simply be that the region of interest is sufficiently complicated to require review in three dimensions. The region of interest may be all or only part of a component, or may be several components.
Three-dimensional models of regions of interest can be obtained by combining multiple two-dimensional images taken from different perspectives, or projections, through the region of interest using a tomosynthesis technique. The resulting three-dimensional model allows a user to inspect any plane through the region of interest, to review a three dimensional image to find defects such as voids. However, in order to obtain a good result from tomosynthesis it is necessary to know with high accuracy the relative positions and orientations of the X-ray source, region of interest and detector. The way in which the two-dimensional images are combined in tomosynthesis relies on this geometric information, as it is required in the mathematical formulas that are used. As a result, existing tomosynthesis systems require high precision mechanical systems for moving the elements of the system i.e. X-ray source, region of interest and detector relative to one another. This need for high precision mechanical equipment makes tomosynthetic x-ray imaging systems much more expensive than two-dimensional x-ray imaging systems.
It would be desirable to provide a relatively inexpensive imaging system that is capable of providing a useful tomosynthetic model of a region of interest.